Technically quite complicated... As soon as you moved the shift, the hydraulic system moved the clutch. Once you had chosen the next speed that same system took care of connecting engine and front wheels. Even the changing of the wheels in the gearbox was done with hydraulic 'power'. It is great fun to drive with a hydraulic gearbox. When you move the shift to the left you start the engin (a key is only used to make contact). Then you push on the brake mushroom (also hydraulic, you can only open a valve by pushing it about 1 mm, and by pushing harder more oil pressure will go to the brakes) and you move the shift forward in first (or forward and to the right in reverse). You take your foot off the brake mushroom, the revs willgo up slightly and the car moves. To go the 2nd you pull the shift back, 3rd is to the right, 4th is further to the right. You just give a bit less gas while shifting up, or a bit more while shifting down, but if everything is well regulated you won't feel anything.

It sounds complicated indeed, when you say to the left or to the right do you actualy mean up and down? otherwise i don't really get how would you move the lever to one side or another, and also, didn't they exist in "normal" with a clutch or in completley automatic?

The lever is positioned almost vertically, so you can move it left and right. The ID models always had normal gearshift, the DS could get a normal, manual gearshift with clutch since February 1963. Since December 1971 there was an option of a fully automatic Bord Warner gearbox for the DS 21 and DS 23, the same as used in Jaguar XJ6 and Rover 3.5 litre. This automatic box was relatively expensive and not very reliable...